1. Field of the Invention
The present invention relates to local area networks (LANs) of the Ethernet type, and more particularly, to a method for improving fairness in use of a network when collision occurs in an Ethernet LAN.
2. Background of the Related Art
Under Carrier Sense Multiple Access with Collision Detection (CSMA/CD), all nodes on the network have equal priority of access. Any node or station wishing to transmit a packet must first make sure that the channel is clear during inter packet gap (IPG) before beginning to transmit the packet. The IPG is 9.6 xcexcs on the network of 10M while 0.96 xcexcs on the network of 100M. However, if a first node that has started transmitting detects a xe2x80x9ccollisionxe2x80x9d with a transmission from another node, the first node continues transmitting for a short time to make sure that all nodes wishing to transmit will detect the collision. Every other node detecting the collision also continues to transmit for a short time.
Next, channel access steps of a typical Ethernet LAN will be described with reference to FIG. 1.
As shown in FIG. 1, among m nodes, i.e., a first node S1 to mth node Sm, the first node transmits a first packet, and transmits a second packet after delay by IPG. Thereafter, the first node transmits a third packet after delay by IPG. The mth node transmits a fourth packet after delay by IPG. At this time, since both the first node and the mth node transmit their own packet after transmission of the second packet, collision occurs after xc2xd of slot time has elapsed.
The nodes involved in the collision wait for the required interpacket delay and then select, and therefore usually different, delay times, referred to as backoff times, before trying transmission of the same packet again.
When a transmission attempt has terminated due to a collision, it is retried by the transmitting node, after a selected backoff time, until either the transmission is successful or a maximum number of attempts have been made and all have terminated due to collisions.
A related art (the IEEE 802.3 standard) defines a collision backoff procedure referred to as xe2x80x9ctruncated binary exponential backoff.xe2x80x9d
FIG. 2 shows channel capture when a related art standard backoff algorithm is used.
The backoff time is selected by each node as an integral multiple of the xe2x80x9cslot time,xe2x80x9d which is the maximum round-trip propagation time for the network, i.e., the time to propagate a data packet from one end of the network to the other, and back. The slot time defined by the network of 10M is 51.2 xcexcs. The number of slot times selected as the backoff time before the nth transmission is chosen as a randomly distributed integer r in the range:
0xe2x89xa6r less than 2k, k=min(n, 10)
wherein n is the number of times of signal retransmission.
The value r for the number of times (1xcx9cn) of signal retransmission is shown in the table 1.
For example, it is assumed that the value n is 1xcx9c10. For the first attempted retransmission (i.e., n=1), k=min(1,10)=1 and 0xe2x89xa6r less than 21. Therefore, r is 0 and 1. At this time, the value r can randomly be selected and the backoff time is selected as 0xc3x97slot time or 1xc3x97slot time.
For the tenth attempted retransmission, k=min(10,10)=10 and 0xe2x89xa6r less than 210. Therefore, r is 0xcx9c1023. At this time, the backoff time is selected as any one of 0xc3x97slot time to 1023xc3x97slot time if the value r is randomly selected.
The slot time for the network of 10M is 52.4 ms.
The above method can provide fairness in case of a variety of nodes.
Signal transmission in case of a small number of active nodes will be described with reference to FIG. 2.
Suppose there are two active nodes, A and B, in a network and they begin transmitting at approximately the same time, resulting in a first collision. They each select backoff times of 0 or 1 slot time, in accordance with the standard backoff algorithm. At this time, if the values r of the nodes A and B are selected as 0 and 1, respectively, the backoff time of the node A is 0xc3x97slot time and the backoff time of the node B is 1xc3x97slot time. Therefore, the node A transmits a signal after delay by IPG. The node B will wait a full slot time before making its first retransmission attempt, but by this time node A has started transmitting a second data packet, i.e. node A is transmitting successive packets back-to-back, with only the required IPG separating them. In other words, the node B waits for the IPG+1xc3x97slot time, and can transmit a signal after delay by IPG following packet transmission of the node A. Therefore, the node A will be able to successfully transmit on its first attempt at retransmission. Subsequently, if the node A continues to attempt packet transmission, another collision occurs between the nodes A and B. At this time, the node A has successfully transmitted packet, in fact, the node A experiences first collision while the node B experiences second collision. In other words, the number of times n of signal retransmission in the node A is smaller than that in the node B. Therefore, there is small probability that the backoff time of the node A is longer than the backoff time of the node B. In other words, the node A is likely to use the network as compared with the node B. The situation may occur, in which the node A continues to transmit packet data while the node B continues to wait packet data.
As described above, in case of the related art packet transmission, channel capture may occur in the LAN in which the number of active nodes is small.
The aforementioned related art method for using network has several problems.
When the number of nodes operated on the network is small, channel capture effect occurs, in which one node continues to either transmit packet data in a back-to-back transmit mode or wait for the required packet data. This reduces fairness in the use of network.
Accordingly, the present invention is directed to a method for improving fairness in use of a network, that substantially obviates one or more of the problems due to limitations and disadvantages of the related art.
An object of the present invention is to provide a method for improving fairness in use of a network, which is suitable for improvement of the network throughput by improving fairness in the use of the network when collision occurs during signal transmission in an Ethernet LAN.
Additional features and advantages of the invention will be set forth in the description which follows, and in part will be apparent from the description, or may be learned by practice of the invention. The objectives and other advantages of the invention will be realized and attained by the structure particularly pointed out in the written description and claims hereof as well as the appended drawings.
To achieve these and other advantages and in accordance with the purpose of the present invention, as embodied and broadly described, a method for improving fairness in the use of a network, according to the present invention, includes the steps of: detecting whether or not collision occurs during packet transmission in a network having a plurality of nodes; successively monitoring X number of times packet transmission state prior to collision of each node; reading monitored value of nodes that have detected collision if collision is detected as a result of collision detection; selecting a value N as a value in the range of 1xe2x89xa6Nxe2x89xa62X in response to transmission and reception state of each node by determining the read state and then determining backoff time, retransmitting a signal of each node after the determined backoff time has elapsed; in case of successive reception, i.e., if the value N is selected as a minimum value (N=1), retransmitting a signal of each node after the backoff time has elapsed, and then monitoring and updating signal transmission and reception state of each node if transmission collision does not occur; performing standard backoff by adding 1 to the value N if transmission collision occurs again during signal retransmission of each node after backoff time corresponding to the case that the value N is selected a minimum value has elapsed; updating network state of each node if transmission collision does not occur during signal retransmission of each node after backoff time corresponding to the case that the value N is selected as 1xe2x89xa6Nxe2x89xa62X greater than the minimum value has elapsed; and determining backoff time by reading node state if transmission collision occurs during signal retransmission of each node after backoff time corresponding to the case that the value N is selected as a value greater than the minimum value has elapsed.
It is to be understood that both the foregoing general description and the following detailed description are exemplary and explanatory and are intended to provide further explanation of the invention as claimed.